Pacemaker-mediated tachycardia over the upper rate limit in a biventricular pacemaker system: what is the mechanism?

摘要

A 51-year-old man with nonischemic dilated cardiomyopathy, a low ejection fraction (25%), and New York Heart Association class III heart failure was referred to our Department for cardiac resynchronization therapy. His electrocardiogram (ECG) showed an atypical right bundle branch block and left posterior hemiblock morphology, with a QRS duration of 160 ms (Fig. 1), and with intra- and inter-ventricular dyssynchrony. A biventricular pacemaker (Stratos LV, Biotronik, Berlin, Germany) was implanted with a bipolar left ventricular (LV) lead positioned in the proximal segment of the anterior vein in the absence of an appropriate lateral or posterior branch. The LV threshold was 3.5 V at 1 ms and the output was programmed to 6 V at 1 ms. One week later, the patient was admitted with sustained regular wide QRS complex tachycardia at 167 beats/min, an almost identical QRS axis in the frontal plane and similar QRS complexes as compared with the baseline nonpaced ECG (Fig. 1). Intracardiac signals during the tachycardia through device interrogation are presented in Figure 2 (left). Prompt termination of the tachycardia was achieved with an iv bolus of adenosine (6 mg). At testing, the atrial and right ventricular (RV) leads showed excellent sensing and capture parameters, with a further increase in the LV threshold, since the implantation and only intermittent capture at the programmed values. Tachycardia with identical morphology to the presenting arrhythmia was reinitiated during the LV capture threshold test (Fig. 3, left). What is the mechanism of this tachycardia?